Watershed management
What a watershed is, why it is the unit for managing land and water, ridge-to-valley treatment, participatory programmes, and a peak runoff sum.
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Rain that falls on a hillside does not respect field boundaries. It runs off the upper slopes, carrying soil, across farms in the middle, into streams and down to valleys and reservoirs. Treating one field at a time cannot solve erosion, floods or falling groundwater; the whole area that drains to a common point must be managed together. That area is a watershed, and watershed management is the integrated planning of land, water, vegetation and people within it.
Watershed management is a standard topic in soil and water conservation, agronomy and rural development courses and exams.
What a watershed is
A watershed (also called a catchment or drainage basin) is the land area that drains to a common outlet, such as a stream, river or lake, bounded by a ridge line (water divide). Watersheds nest within each other:
- large river basins and sub-basins;
- watersheds of thousands of hectares;
- sub-watersheds and micro-watersheds of a few hundred hectares, the usual unit for community planning.
A watershed's shape, slope, drainage density, soils and land use decide how quickly runoff concentrates and how much soil it carries.
Objectives
- Conserve soil: reduce erosion and sedimentation of reservoirs.
- Conserve and harvest water: increase infiltration and groundwater recharge, store runoff for use.
- Increase productivity and incomes from crops, livestock, trees and fish.
- Reduce floods and droughts downstream.
- Restore degraded land and biodiversity.
- Improve livelihoods and equity, including for landless and women.
Ridge-to-valley treatment
Measures are planned from the top of the watershed downward, so that water and soil are held where they fall before they gather speed.
- Upper slopes and ridges: afforestation, grass and shrub cover, continuous or staggered contour trenches, closure to grazing so vegetation recovers.
- Middle slopes and cropland: contour bunds, graded bunds, bench terraces, contour cultivation, strip cropping, agroforestry, conservation agriculture.
- Drainage lines and valleys: gully plugs, loose-boulder and brushwood check dams, masonry check dams, percolation tanks, farm ponds and other water harvesting structures.
The participatory approach
Early watershed programmes were often designed by engineers and implemented by government departments, with little local ownership; structures were neglected once the project ended. Experience led to participatory watershed development, in which communities form watershed committees, plan with technical staff, contribute labour or money, and maintain assets. Key elements:
- Community institutions such as watershed committees and user groups.
- Social fencing: community rules on grazing and tree cutting.
- Attention to equity: landless and women gain through jobs, common land, livestock and enterprises, not only through land improvement that mainly benefits landowners.
- Integration with livelihood activities.
Notable examples
- Loess Plateau, China: one of the world's largest watershed rehabilitation efforts; World Bank-supported projects from 1994 terraced slopes, planted vegetation and restricted grazing over large areas, reducing erosion and raising incomes.
- Villages such as Ralegan Siddhi and Adgaon in Maharashtra, India, transformed through community-led watershed work from the 1970s and 1980s, which inspired national watershed programmes.
- Ethiopia's community watershed programmes since the 2000s, rehabilitating highland catchments.
Measuring success
Indicators include soil loss and sediment yield, runoff and groundwater levels, cropped area and cropping intensity, yields and incomes, vegetation cover, migration and equity of benefits. Hydrological monitoring at the outlet is essential but often neglected.
Estimating peak runoff
Designing a check dam spillway or waterway needs the peak runoff rate from the area above it. For small watersheds, the rational method is widely used:
where Q is peak runoff (m3 per second), C the runoff coefficient (0 to 1, higher for bare, steep or clayey land), I the rainfall intensity (mm per hour) for a storm lasting as long as the time of concentration (the time for water to travel from the farthest point to the outlet), and A the area (hectares).
Peak runoff from a small watershed
A 50 ha micro-watershed of cultivated land on moderate slopes has a runoff coefficient C = 0.4. The design storm intensity for its time of concentration, at the chosen return period, is 60 mm per hour.
- Q = 0.4 × 60 × 50 ÷ 360 = 1,200 ÷ 360 ≈ 3.3 m3 per second.
If the upper slopes are revegetated and bunded so that C falls to 0.25, Q = 0.25 × 60 × 50 ÷ 360 ≈ 2.1 m3 per second, about 37 per cent less. Smaller peaks mean less erosion, smaller and cheaper structures downstream, and more water soaking into the soil.
Key points
- A watershed is the area draining to a common outlet, bounded by a ridge line; micro-watersheds are the usual planning unit.
- Objectives: conserve soil and water, raise productivity, reduce floods and droughts, restore land and improve livelihoods.
- Ridge-to-valley treatment: vegetation and trenches above, bunds and terraces on cropland, check dams and ponds in drainage lines.
- Participatory approaches with community institutions, social fencing and equity for landless and women make results last.
- Rational method: Q = C I A ÷ 360 (m3 per second, with I in mm per hour and A in hectares).
What the exams ask
- School-leaving and entrance papers: meaning of watershed, objectives of watershed management, and conservation measures.
- Degree and research-fellowship entrances: classification of watersheds, ridge-to-valley treatment, participatory approaches, evaluation indicators, and the rational method.
- Agricultural engineering and professional examinations: watershed planning and design, hydrological monitoring, structure design and project appraisal.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why start treatment at the ridge?
Water and soil move downhill. Slowing runoff at the top keeps it from gathering speed and volume, protecting the structures and fields below. Building check dams first, without upstream treatment, often leads to them filling with silt.
Who benefits from watershed projects?
Landowners in the treated area usually benefit most from higher water tables and yields. Programmes must deliberately include landless households, for example through jobs, common land and livestock, to share benefits fairly.
How long does it take to see results?
Some effects, such as reduced runoff and better water in wells, can appear within one or two monsoon seasons. Vegetation recovery and soil improvement take several years.
Sources
- FAO. Watershed Management Field Manual. FAO Conservation Guide 13, 1985–1988.
- Kerr, J. Watershed Development Projects in India: An Evaluation. IFPRI Research Report 127, 2002.
- World Bank. Restoring China's Loess Plateau. World Bank, 2007.
- Brooks, K. N., Ffolliott, P. F. and Magner, J. A. Hydrology and the Management of Watersheds, 4th edition. Wiley-Blackwell, 2013.
- Suresh, R. Soil and Water Conservation Engineering. Standard Publishers, latest edition.
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